Photoelectric conversion devices, such as, solar cells, photosensors, light-emitting devices, photodiodes and optical memories, can comprise perovskite or organic materials serving as the photoelectric conversion components. Those are expected as low-cost photoelectric conversion devices because their layers can be formed by inexpensive coating processes.
Perovskite solar cells, which comprise perovskite materials serving as the photoelectric conversion components, have been recently hoped to be low-cost and highly efficient solar cells. When perovskite solar cells are manufactured by a wet-coating process, a roll-to-roll method can be adopted. In the roll-to-roll method, a substrate loaded with layers of the solar cell is transferred around each roll and hence needs to be flexible.
For the flexible substrate, a polymer film is suitably employed. In addition, because a polymer film is generally lighter in weight than a common glass substrate, the resultant solar cell can be potentially installed on curved or poor load-bearing roofs of factories and the like, on which it has been difficult to install conventional solar cells. Thus, there is possibility that perovskite solar cells comprising polymer film substrates may extend the market of solar cells.
A perovskite solar cell generally comprises such components as a substrate/a transparent electrode/a first intermediate layer/a perovskite layer (photoelectric conversion layer)/a second intermediate layer/and a counter electrode. Examples of materials for the transparent electrode include indium-tin oxide (ITO) and fluorine-doped tin oxide (FTO). The electrode of ITO has lower sheet resistance and smaller surface roughness than that of FTO, and hence is advantageous to enhance the efficiency.
The first intermediate layer is made of organic materials, inorganic materials or composite materials of both. For example, it is known that high efficiency is realized by adoption of a perovskite solar cell comprising a ITO transparent electrode and a first intermediate layer made of polyethylenedioxythiophene:polystyrenesulfonic acid (PEDOT:PSS). This can be explained by the following three reasons.
First, that is because the ITO film electrode has high light transmittance, low sheet resistance and small surface roughness.
Second, that is because PEDOT:PSS has excellent electroconductivity and high light transmittance; and thirdly, that is because the work functions of ITO and PEDOT:PSS are positioned at such near energy levels to the valence band of the perovskite material that carriers can migrate efficiently.
The ITO film can be formed on a glass substrate at a high temperature because glass generally has high heat-resistance. Because of the film-formation at a high temperature, the ITO film thus formed is crystalized. On the other hand, however, if a polymer film is adopted as the substrate, the ITO film must be formed at a low temperature because the polymer film substrate may be damaged at a high temperature. In this case, since formed at a low temperature, the ITO film is in an amorphous phase.
The ITO film is then coated with PEDOT:PSS, which is PEDOT doped with acidic PSS and hence has strong acidity (a low pH value). However, it is known that, although having acid resistance in a crystal phase, ITO did not have chemical resistance and hence is vulnerable to acid in an amorphous phase. Accordingly, if acid resistance is regarded as important, the ITO film needs to be formed at such a high temperature as to be crystalized and therefore the substrate cannot be a polymer film. Further, the crystalized ITO film is also unfavorable in view of flexibility. On the other hand, when the first intermediate layer is formed on the amorphous ITO film by a wet-coating process, the solvent of the coating solution must be selected in consideration of poor chemical resistance of the amorphous ITO film so that the solvent may not damage the ITO film. This requirement narrows down selection of formable first intermediate layers, and hence seriously limits development of perovskite solar cells.
In order to reduce the damage of ITO in the structure of a polymer substrate/an amorphous-phase ITO film/a PEDOT:PSS layer of low pH value, it is proposed to crystalize the surface of the amorphous-phase ITO film by laser annealing. The crystalized surface functions as a protective layer, and hence can reduce possibility that the amorphous-phase ITO film may be damaged by the PEDOT:PSS layer of low pH value. However, the crystalized surface must be thickened enough to serve as a protective layer, and accordingly it is necessary to increase the laser power and the exposure time thereof. Consequently, the production cost increases. In addition, if comprising a thick crystalized surface, the ITO film may lose flexibility. Further, if large-area ITO films are intended to be annealed, laser instruments for the annealing treatment needs to be enlarged and the number thereof needs to be increased although they are expensive. Accordingly, the production cost further goes up.